Sub-10 fJ/bit radiation-hard nanoelectromechanical non-volatile memory

With the exponential growth of the semiconductor industry, radiation-hardness has become an indispensable property of memory devices. However, implementation of radiation-hardened semiconductor memory devices inevitably requires various radiation-hardening technologies from the layout level to the system level, and such technologies incur a significant energy overhead. Thus, there is a growing demand for emerging memory devices that are energy-efficient and intrinsically radiation-hard. Here, we report a nanoelectromechanical non-volatile memory (NEM-NVM) with an ultra-low energy consumption and radiation-hardness. To achieve an ultra-low operating energy of less than 10 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{{{{{\rm{fJ\; bit}}}}}}}^{-1}$$\end{document}fJ bit−1, we introduce an out-of-plane electrode configuration and electrothermal erase operation. These approaches enable the NEM-NVM to be programmed with an ultra-low energy of 2.83 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{{{{{\rm{fJ\; bit}}}}}}}^{-1}$$\end{document}fJ bit−1. Furthermore, due to its mechanically operating mechanisms and radiation-robust structural material, the NEM-NVM retains its superb characteristics without radiation-induced degradation such as increased leakage current, threshold voltage shift, and unintended bit-flip even after 1 Mrad irradiation.


Table of contents
shows the structure and operation mechanisms of the conventional nanoelectromechanical non-volatile memory (NEM-NVM). It consists of a deformable beam and two in-plane electrodes for program and erase operations. For program operation, when an operating voltage is applied between the deformable beam and program electrode, the beam is deflected laterally by the electrostatic force generated from the program electrode and is held in contact by an adhesion force. In the case of the erase operation, similarly, the deformable beam is deflected in the opposite direction by the erase electrode. Supplementary Fig. 1b, c show cross-sectional transmission electron microscope (TEM) images of the initial state and the programmed state of the conventional NEM-NVM, respectively. This NEM-NVM was fabricated using a back-end-of-line process of the TSMC company, which is similar to the fabrication method used in previous studies 1,2 . The in-plane electrode configuration makes the fabrication process very easy and simple. Supplementary Fig. 1d shows the fabrication method of the conventional NEM-NVM with an in-plane electrode configuration. First, a sacrificial layer and structural material are sequentially deposited on the Si wafer, and then a deformable beam and two in-plane electrodes are patterned through a photolithography and an etching process. Finally, the sacrificial layer is removed to release the deformable beam.
The conventional NEM-NVM has several issues related to the energy-efficiency. The first problem is the in-plane electrode configuration. The air gap and width of the deformable beam, the most important factors of the operating energy, are determined by the resolution of the photolithography tools. Therefore, this limits the effective scaling of the operating energy. The second problem is the electrostatic erase operation. It inevitably doubles the actuation air gap distance, which causes an increase in the operating energy and voltage starting with the second operation. Therefore, the conventional NEM memory devices typically have a long beam shape to reduce the operating energy and voltage. Additionally, the inclined sidewall of the in-plane 4 electrode configuration is the main reason for performance degradation. As shown in Supplementary Fig. 1b, the side wall, where the contact occurs, is fabricated to be inclined due to the limitation of the patterning process. The inclined sidewall makes it difficult to implement conformal contact between the beam and operation electrodes. This non-conformal contact can cause performance degradation such as high contact resistance, low reliability, and reduced retention performance.

Supplementary Note 3: Programming energy calculation
The energy consumed in the program operation of the NEM-NVM is calculated by the following equation 3 .
(1) (2) where is the capacitance between the top and bottom electrodes, is the operating voltage, is the mechanical spring constant, is the actuation air gap, is the total mass of the top electrode, is the velocity related to the movement of the top electrode, and is the viscosity of quasi-ideal gas at standard temperature and pressure. Since the kinetic energy and energy lost in damping are relatively very small, the electrical energy and mechanical spring energy are the dominant values in the total operation energy. The capacitance and operating voltage are calculated by the following equation 3 . Therefore, the total operation energy is determined by the actuation air gap and stiffness as shown in Fig. 1d.

Supplementary Note 4: Analysis of the adhesion force between metallic surfaces
Generally, van der Waals (vdW) force and metallic bonding force are the dominant forces among the various interfacial forces between rough surfaces. Therefore, separation distance ( and the actual contact area ( ) between the tungsten surfaces are needed to calculate the adhesion force. The van der Waals force and metallic bonding force between rough surfaces are calculated by the following equations 4-7 .
where is the hamarker constant, is function representing the retardation of with separation distance, is the characteristic wavelength constant, is the radius of curvature of asperities, is the work of adhesion, and is the hardness. However, it is very challenging to accurately measure the separation distance and the actual contact area because the contact surfaces are buried in the two bodies. Thus, we used the well-established adhesion

Supplementary Note 5: An analytical stiffness model and operation design process
Precise design of the mechanical structure is very important for reliable operation of the NEM-NVM, which stores information through mechanical movement. Therefore, we used both Castigliano's theorem and FEM simulation to model the mechanical structure, and analyzed the parameters for reliable program and erase operations ( Supplementary Fig. 4, 5).
Finally, the optimally designed NEM-NVM through the mechanical analysis is shown in Supplementary Fig. 6.

Supplementary Note 7: Analysis of the program operation
We need to know the actual air gap and the capacitance in the initial state to accurately calculate the programming energy. Through the measured operating voltage (9.9 V) as shown in Fig. 3a, the actual air gap was estimated using an analytical stiffness model (from Supplementary Note 5) and FEM simulation, respectively ( Supplementary Fig. 8a).

fJ bit
We also measured the I-V dual sweep curves of the NEM-NVM as shown in Supplementary   Fig. 8b. When the applied voltage was increased above the threshold (program) voltage and then swept back to zero, 'On' current maintained the compliance value of 100 nA, but gradually decreased as the applied voltage decreased. These results indicate that the NEM-NVM successfully operated in a non-volatile manner via delicate engineering of the mechanical structure as we described in Supplementary Note 5. Furthermore, due to the reliable fabrication using the CMOS manufacturing processes, the memory devices also show very uniform contact characteristics.

Supplementary Note 8: Estimation of the erase temperature
A resistance-temperature detector (RTD) method and time-dependent FEM simulation were conducted to estimate the erase temperature of the NEM-NVM. First, the resistance of the pipeclip spring was measured according to the temperature on an external hotplate. The exact temperature of the pipe-clip spring on the heated hotplate was measured by an infrared thermometer. As shown in Supplementary Fig. 9a, the resistance of the pipe-clip spring has a linear correlation with the temperature due to the stable thermal property of tungsten 8

Supplementary Note 9: Stability of the NEM-NVM in harsh environments
Thanks to the radiation-robust structural material and mechanically operating mechanisms, the NEM-NVM shows excellent radiation-hardness without additional rad-hard processing and complex circuit configurations. The electrical conductivity of nano-patterned tungsten and I-V characteristics of the NEM-NVM before and after exposure to the high-energy radiation was shown in Supplementary Fig. 10, 11